The invention relates to the suppression and control of slug flow in a multi-phase fluid stream. More particularly, the invention relates to a method and apparatus for suppressing and controlling liquid slugs and gas surges in a stream of multi-phase fluid flowing through a pipeline or a flowline system, which may include a riser section and a gas/liquid separator or slugcatcher located downstream of the pipeline outlet or the riser section.
In the oil and gas industry it is common practice to transport a multiphase fluid containing crude oil or condensate, water and gas from a well through a single pipeline system to a process facility. For example, in case of offshore oil production, crude oil, production water, and associated gas are generally simultaneously transported through a single subsea pipeline to gas/liquid separating equipment located onshore or on an offshore platform. Several flow regimes are known to occur in such a stream of multiphase fluid, including stratified flow, core flow, mist flow, and slug flow. Of these flow regimes, slug flow is generally to be avoided as it consists of alternating batches of liquid (termed slugs) and gas surges. Under certain flow conditions growth of liquid slugs may easily occur, thus leading to severe slugging characterized by a flow pattern of alternating production starvation (no flow), large liquid slugs, and strong gas surges at the exit of the flowline system. Large liquid slugs can also be generated by operational changes, e.g. the increase of the fluid production during the start-up of a pipeline. Supplying such an alternating pattern of liquid slugs and gas surges to a gas/liquid separator strongly reduces the efficiency of the separator, as the gas/liquid separator must be operated with acceptable pressure fluctuations and should deliver an acceptably low liquid content in the gas outlet conduit and an acceptably low gas content in the liquid outlet conduit.
A method for preventing slug growth in a pipeline system during simultaneous transportation of a gas and a liquid through the pipeline system is disclosed in Oil and Gas Journal, Nov. 12, 1979. In this known method a valve is arranged at the top of a riser, which valve is manually or automatically regulated so as to minimize the pipeline pressure upstream of the riser and to minimize the differential pressure fluctuations in the riser. Transmitters are used to transmit pressure signals for regulating the valve, which transmitters are installed at a subsea part of the pipeline system. This known method is based on the assumption that severe slugging only occurs in pipelines having a section of downward inclination when seen in the direction of flow and that slug growth can be prevented by regulating the volumetric fluid flux as a function of fluid pressure variations.
JP63274408A2 discloses a separator control apparatus which adjusts a valve in a gas outlet of a gas liquid separator by adding the outputs of a supersonic multiphase flowmeter in the inlet of the separator and of a manometer within the separator to exert a constant pressure in the separator.
EP410522B1, assigned to Shell, and incorporated by reference herein in the entirety, discloses a method of preventing slug growth in a stream of multiphase fluid flowing from a flow line into a gas/liquid separator by providing a means of fluid control to manipulate the fluid flow rate. This method comprises measuring the liquid flow rate in the liquid outlet of the separator and the gas flow rate in the gas outlet thereof, determining the fluid flux defined as the sum of the liquid flow rate and the gas flow rate, and operating the fluid flow rate control means so as to reduce a variation of the fluid flux. A drawback of EP410522B1 and of the method disclosed in JP63274408A2 is that measurement of the sum of the liquid and gas flow rates in a multiphase fluid stream is difficult and requires complex measuring equipment.
U.S. Pat. Nos. 5,256,171; 5,377,714; 5,494,067; 5,544,672 and 5,711,338 disclose slug suppression methods in which the liquid level in a gas-liquid separation vessel is held as constant as possible. A disadvantage of these methods is that said liquid level is not always the best control parameter and that occasionally, e.g. during start-up still manual control or intervention is required.
The method of EP00767699B1, assigned to Shell, and incorporated by reference herein in the entirety, comprises the steps of:
a) measuring at least one control variable selected from the group consisting of the liquid level in the separator, the liquid flow rate in the liquid outlet of the separator, the gas flow rate in the gas outlet of the separator, the sum of the liquid flow rate in the liquid outlet and the gas flow rate in the gas outlet, and the fluid pressure at or near the separator; and
b) operating said liquid flow rate control means essentially on the basis of said measurements so as to reduce a difference between said liquid level and a selected reference value of the liquid level, and so as to reduce a difference between said control variable and a selected reference value of the control variable.
Field experience with the method according to EP00767699B1 has demonstrated that this method significantly suppresses slug flow in a multi-phase fluid stream, but that, due to the origin of the liquid slugs, some slugs were suppressed to a more limited extent than other slugs. In addition, the method of EP00767699B1 requires significant monitoring and is not operated automatically.
It would be very desirable in the art if a method were available for slug suppression that could be operated automatically. The method and apparatus of the present invention provides such a method for slug suppression and control that can be operated without human intervention using a control system with feedback and selected set-points to trigger adjustments in the system.
In accordance with the foregoing the present invention is an improved method for suppressing and controlling liquid slugs and gas surges in a stream of multiphase fluid flowing from a flowline into a gas/liquid separator which has a liquid outlet provided with a liquid flow control valve, and a gas outlet provided with a gas flow control valve, the method comprising:
1) measuring at least one control variable selected from the group of the liquid level LLIQ in the separator, the liquid flow rate QL in the liquid outlet, the gas flow rate QG in the gas outlet, the sum (QL+QG) of the liquid flow rate in the liquid outlet and the gas flow rate in the gas outlet, and the fluid pressure (P) at or near the separator;
2) Inputting said measurements to a control system with a means for receiving said values as a mathematical number and responding to preset numbers;
3) Said control system adjusting said liquid and gas flow control valves to reduce a difference between a selected control variable (QL+QG, QL, QG, P, LLIQ) and a pre-set reference value of the selected control variable; and
4) Changing the control variable (QL+QG, QL, QG, P, LLIQ) from time to time automatically if one or more control variables reaches a pre-set value.
The Slug Suppression Device of the present invention (Hereafter referred to a SSD) provides a reliable solution to the slugging problem. The implementation of a SSD results in a stabilized production of gas and liquid. The apparatus comprises a small separator, xe2x80x9cmini-separatorxe2x80x9d, positioned between the pipeline outlet and the normal first stage separator. The SSD functions as a control valve with separate control valves for each of the two phases present in the system and with the use of conventional measuring equipment for mass flows, pressure, and level.
The control strategy of the SSD uses two modes: Total volumetric flow control and liquid control. In the total volumetric control mode the liquid valve is controlled to maintain a level set-point. In addition, the gas valve is controlled to maintain a total volumetric flow set-point. The actual flow rates are measured by the flow meters after the liquid and gas control valves. The sum of the output of the flow meters is the variable to be controlled. The set-point of the total volumetric flow is adjusted by a pressure controller in combination with certain equations. These adjustments depend on the pressure and the set-point of the pressure in the SSD and factors which depend on the size of the flowline.